The basic tools of physical science, sometimes
called the laws of nature, comprising mathematical equations that
govern the behaviour of matter (in the form of elementary particles)
and energy according to various fundamental interactions. Experimental
results obtained in the laboratory or through observations of natural
physical processes can be used to infer mathematical rules which
describe these data. Alternatively, a theory may be created first as
the result of a hypothesis or physical principle, which receives
experimental confirmation only at a later stage.

As our understanding evolves, seemingly disparate physical laws become
unified in a single overarching theory. The tendency of apples to fall
to the ground and the tendency of the Moon to orbit the Earth were
thought to be different things before the emergence of Isaac Newton's
laws of motion and his theory of gravity. This theory was thought to be
complete until the work of Albert Einstein, who showed that it was
lacking in many aspects. A more complete (and much more mathematically
intricate) theory of general relativity took the place of Newton's
theory in 1915. In modern times, physicists are trying to unify general
relativity with the rest of the theory of fundamental interactions into
a theory of everything, a single mathematical formula from which all of
physics can be derived (see also grand unified theory, string
theory, supersymmetry).

Although this ambitious programme is far from complete, similar
developments have occurred throughout the history of science, to the
extent that
the exact form of laws of physics available to working scientists
changes significantly with time. Nevertheless, the task of a physical
cosmologist remains the same: to take whatever laws are known (or
whichever hypotheses one is prepared to accept) and work out their
consequences for the evolution of the Universe at large. This is what
cosmologists have done all down the ages, from Aristotle to the
present generation of early-Universe cosmologists.

But there are deep philosophical questions below the surface of all
this activity. For example, what if the laws of physics were different
in the early Universe - could we still carry out meaningful
research? The answer to this is that modern physical theories
actually predict that the laws of physics do change, because of the
effects of spontaneous symmetry-breaking. At earlier and earlier
stages in the Big Bang theory, for example, the nature of the
electromagnetic and weak interactions changes so that they become
indistinguishable at sufficiently high energies. But this change in
the law is itself described by another law: the so-called electroweak
theory. Perhaps this law itself is modified at scales on which grand
unified theories take precedence, and so on right back to the very
beginning of the Universe.

Whatever the fundamental rules may be, however, physicists have to
assume that they apply for all times since the Big Bang. It is merely
the low-energy outcomes of these fundamental rules that change with
time. By making this assumption they are able to build a coherent
picture of the thermal history of the Universe which does not seem to
be in major makes the assumption reasonable, but does not prove it to
be correct.

Another set of important questions revolves around the role of
mathematics in physical theory. Is nature really mathematical, or are
the rules we devise merely a kind of shorthand to enable us to
describe the Universe on as few pieces of paper as possible? Do we
discover laws of physics, or do we invent them? Is physics simply a
map, or is it the territory itself?

There is also another deep issue connected with the laws of physics
pertaining to the very beginning of space and time. In some versions
of quantum cosmology, for example, we have to posit the existence of
physical laws in advance of the physical universe they are supposed to
describe. This has led many early-Universe physicists to embrace a
neo-Platonist philosophy in which what really exists is the
mathematical equations of the (as yet unknown) theory of everything,
rather than the physical world of matter and energy. But not all
cosmologists get carried away in this manner. To those of a more
pragmatic disposition the laws of physics are simply a useful
description of our Universe, whose significance lies simply in their
very usefulness.

FURTHER READING:

Barrow, J.D., The World Within the World (Oxford University Press,
Oxford, 1988).
Barrow, J.D., Pi in the Sky (Oxford
University Press, Oxford, 1992)